Tiny Tubular Generators

To the many properties of carbon nanotubes, we can now add electrical generation

29 March 2010—A team of scientists led by chemical engineering professor Michael Strano of MIT may have stumbled on a new way to produce electricity using carbon nanotubes, as they explain in a recent issue of Nature Materials.

Since their discovery in the early 1990s, carbon nanotubes have turned out to be remarkably versatile for research applications. These thin, cylindrical carbon molecules, typically nanometers in diameter, have a remarkably large number of electrical and structural properties. They are used to reinforce high-end tennis rackets and bicycle handlebars, to craft Lilliputian nanomotors, and to modulate signals in electronics. Potential applications include transistors for computer circuits (demonstrated by IBM), computer memories (being developed by Nantero), and solar cells. In the past couple of years, scientists have also demonstrated loudspeakers and a tiny ”nanoradio” made with nanotubes.

Strano and his collaborators coated multiwalled carbon nanotubes with cyclotrimethylene trinitramine (CNT), a chemical fuel. When they shot a laser beam or produced a high-voltage electrical spark at one end of a CNT-coated nanotube bundle, the CNT ignited, and a speedy thermal wave was created that traveled through the nanotubes much as a flame travels through a fuse. This wave in turn produced a burst of electricity by pushing electrons through the nanotubes in front of it. (Electricity is produced by the movement of electrons.)

This effect has not been observed before and it’s generating quite a bit of interest among scientists and engineers. ”Nanotubes are usually regarded as uninteresting for thermoelectric energy conversion because of their very large thermal conductivity. Paradoxically, it is precisely this good thermal conductivity that appears to enable the effect,” says Natalio Mingo, a senior scientist at the French Atomic Energy Commission’s Laboratory for Innovation in New Energy Technologies and Nanomaterials, in Grenoble, France.

Strano says that as the CNT burns, the heat is directed into the nanotube bundle in a ”wicking” effect, so that it travels 10 000 times as fast as it can in the fuel itself. ”Nanotubes are extremely good at conducting heat along their length,” he explains. ”They can conduct heat more than a factor of 100 times faster than a metal.”

The entire phenomenon is a combination of combustion and electrical power generation, which Strano calls ”thermopower waves.” He says that the electrical energy produced by the nanotubes is 100 times as great as what would be produced in a lithium ion battery if you took an equivalent weight of the battery. The exact mechanism by which the electricity is produced is still not properly understood.

”Unlike a battery or supercapacitor, there is zero self-discharge with this approach. Plus, it works well for powering small things, since the power density is very large,” Strano says.

Combustion waves have been studied for more than a century. Strano and his collaborators predicted recently that combustion waves could be guided by a nanotube or nanowire, which in turn could push an electrical current along in front of it. However, in the experiments they performed, they reported that ”the amount of power released is much greater than predicted.”

”There’s something else happening here,” he says. ”We call it ’electron entrainment,’ since part of the current appears to scale with wave velocity.”

Whatever the mechanism, independent experts are intrigued. ”Even though the demonstrated efficiencies are still lower than 1 percent, the experiment is quite spectacular,” says Mingo.

Strano envisions such potential applications as transponders, beacons, and actuators in cases where a burst of energy is needed. But the jury needs more evidence. ”It will be trickier to motivate an application, especially because scaling degrades the figures of merit for the system in all of the proposed application areas,” says electrical engineering professor John Kymissis of Columbia University, who is impressed with the experiment.

Strano thinks that thermopower could be used to create better fuel cells. ”The conventional fuel cell has been around since the 1800s, but corrosive fuels and catalytic deactivation have been a hurdle,” he says. ”Thermopower waves could be a very simple alternative.”